1. Field of the Invention
This invention relates generally to solar power control systems, and in particular, to an efficient system and method for applying solar-generated power to refrigeration.
2. Description of the Related Art
Two billion people live without electricity. They represent a market for various solar powered systems such as stand-alone power systems and small capacity solar refrigerators. Efforts have been made to develop stand-alone photovoltaic (PV) power systems that provide lighting and power for small devices such as radios and small televisions. For example, such systems may include a solar panel, a battery, and a low wattage fluorescent light. Solar refrigerators, however, represent a bigger challenge.
Previous attempts to produce a marketable solar refrigerator have been largely unsuccessful. For example, consider the following patents:
In U.S. Pat. No. 4,126,014, Thomas Kay discloses an absorption refrigeration system powered by a heated fluid from a solar panel.
In U.S. Pat. No. 5,501,083, Tae Kim discloses an AC-powered air conditioner having a solar panel for backup electrical power.
In U.S. Pat. No. 5,497,629, Alexander Rafalovich discloses the use of solar power in an air conditioning system to pump heat from an indoor space to a thermal store.
In U.S. Pat. No. 5,685,152, Jeffrey Sterling discloses using a heated medium from solar collectors to produce a cold thermal store and mechanical energy to pump heat from an indoor space to the cold thermal store.
Kay""s refrigeration system provides no means to maintain refrigerator operation in the absence of sunlight (e.g. at nighttime or on overcast days). As the air conditioning systems are largely unsuited for even small capacity refrigerators or freezers, no attempt has been made to scale these systems to produce a commercializable solar refrigerator.
Accordingly, it is desirable to provide an efficient, inexpensive, commercializable small capacity solar refrigerator which can operate for several days in the absence of sunlight. As batteries are often expensive and require regular maintenance, it would further be desirable to provide such a solar refrigerator which does not require batteries.
A solar powered vapor compression refrigeration system is made practicable with thermal storage and novel control techniques. In one embodiment, the refrigeration system includes a photovoltaic panel, a capacitor, a compressor, an insulated enclosure, and a thermal reservoir. The photovoltaic (PV) panel converts sunlight into DC (direct current) electrical power, some of which is stored in the capacitor. The capacitor provides additional current during compressor start-up, and thereafter acts to smooth out variations in the power voltage. The power from the PV panel and capacitor drives the compressor to circulate refrigerant through a vapor compression refrigeration loop, thereby extracting heat from the insulated enclosure. The thermal reservoir is situated inside the insulated enclosure and includes a phase change material. As heat is extracted from the insulated enclosure, the phase change material is frozen. Thereafter the thermal reservoir is able to act as a heat sink to maintain the temperature of the insulated enclosure for an extended period in the absence of sunlight.
This conversion of solar power into stored thermal energy is optimized by a compressor control method that effectively maximizes the compressor""s usage of available energy. A controller monitors the rate of change of the smoothed power voltage to determine if the compressor is operating below or above the maximum available power, and adjusts the compressor speed accordingly. In this manner, the compressor operation is continuously adjusted to convert substantially all available solar power into stored thermal energy.